Ethylene glycol can be prepared by a variety of routes including from sugars, e.g. monosaccharides, disaccharides or syrups, via fermentation and hydrogenolysis processes, or by hydroformylation of formaldehyde.
The fermentation route is a five-step process wherein glucose is fermented to ethanol and carbon dioxide, followed by conversion of ethanol to ethylene, ethylene to ethylene oxide and ethylene oxide to ethylene glycol. One disadvantage of this method is that per mole of glucose fermented, two moles of carbon dioxide are produced together with two moles of ethanol; this has the effect that a theoretical maximum 67% of the carbon present in the glucose can be transformed to ethanol.
The hydrogenolysis route is a two-step process wherein glucose is reduced to sorbitol followed by hydrogenolysis of sorbitol to ethylene glycol, as illustrated by U.S. Pat. No. 6,297,409 B1 and US 2008/0228014 A1. Significant quantities of propylene glycol, compared to ethylene glycol, are formed via the hydrogenolysis process. Additionally, the amount of catalyst used is significant and appears difficult to regenerate in order to reuse. Furthermore, the byproducts formed, in particular butanediols, are difficult to separate from the desired product. In particular, the industrially favorable method of distillation for separation (purification) purposes is extremely difficult to apply as the byproducts have very similar boiling points to the final product, and the desired product may react further, as illustrated in US2014/0039224 A1 and U.S. Pat. No. 5,393,542 B1.
The hydroformylation route is a two-step process wherein glycolaldehyde is prepared from formaldehyde, carbon monoxide and hydrogen, followed by hydrogenation of the glycolaldehyde to ethylene glycol, as illustrated in U.S. Pat. No. 4,496,781 B1. There appears to be several extraction steps present in order to separate formaldehyde from glycolaldehyde and proceed with the hydrogenation reaction.
It is known that sugars may be subjected to thermolytic fragmentation to obtain a fragmentation product composition comprising oxygenate compounds such as glycolaldehyde (U.S. Pat. No. 7,094,932 B2); the crude fragmentation product composition comprises C1-C3 oxygenate compounds, including formaldehyde, glycolaldehyde, glyoxal, pyruvaldehyde and acetol. The main product of this reaction is glycolaldehyde [U.S. Pat. No. 7,094,932 B2]. Water is the solvent of the reaction.
It is also known that pure glycolaldehyde may be hydrogenated to ethylene glycol. U.S. Pat. No. 4,200,765 B1 discloses hydrogenation of glycolaldehyde under severe conditions: at high pressure [3000 psi (ca. 202 bar)], high temperature [150° C.], and with an organic solvent [N-methyl pyrrolidine] in the presence of a palladium on carbon [Pd/C] catalyst for an extended period [5 h]. U.S. Pat. Nos. 4,321,414 B1 and 4,317,946 B1 disclose the hydrogenation of glycolaldehyde with a homogenous ruthenium catalyst and U.S. Pat. No. 4,496,781 B1 discloses a continuous flow hydrogenation at low pressure [500 psi (ca. 35 bar)], high temperature [160° C.] with a ruthenium on carbon catalyst [Ru/C] in ethylene glycol and trace acetonitrile as solvent.
As illustrated, the two steps, pyrolysis of glucose to obtain, inter alia glycolaldehyde, and hydrogenation of pure glycolaldehyde in the liquid phase, appear to be independently feasible. However, in order for the pyrolysis product composition to be hydrogenated, laborious separation processes are employed to remove formaldehyde from the pyrolysis product composition to avoid formaldehyde poisoning of the hydrogenation catalysts [U.S. Pat. No. 5,210,337 B1]. U.S. Pat. No. 5,393,542 B1 discloses an exemplary purification process comprising multiple distillation steps followed by a solvent-induced precipitation to obtain a glycolaldehyde composition free of formaldehyde.
With regard to hydrogenation of glycolaldehyde, although there is the provision of suitable reaction conditions to obtain a high yield in organic solvents, the reaction with water as a solvent appears to be less successful. U.S. Pat. No. 5,393,542 B1 discloses thermal degradation of glycolaldehyde (2-hydroxyacetaldehyde) when subjected to temperatures of 90° C. or higher and where water is the solvent.
EP 0 002 908 B1 discloses the variation in yield (conversion and selectivity) of the hydrogenation of glycolaldehyde with the use of various catalysts in an aqueous solution at 110° C.: Raney Nickel [100% conversion 49.4% selectivity], 10% Pd/C [62% conversion, 61% selectivity] and 10% Pt/C [100% conversion, 73% selectivity]. A problem with catalysts used in liquid water is the strain on the catalyst. However, mild reaction conditions are favorable in order to ensure longevity of the catalyst on an industrial scale.
The choice of catalyst may affect the decomposition of glycolaldehyde when in the presence of the catalyst; U.S. Pat. No. 5,210,337 B1 discloses the problem of glycolaldehyde ‘unzipping’ to form formaldehyde and consequently poisoning the hydrogenation catalyst. It is also possible that glycolaldehyde may self-condense or condense with another C1-C3 oxygenate compound, also illustrated in U.S. Pat. No. 5,210,337 B1. Accordingly, both the choice of catalyst and the stability of the glycol product may affect the degree of reduction of the glycolaldehyde. E.g. some catalysts may reduce the glycolaldehyde to ethanol or ethane, i.e. over reduce the glycolaldehyde.
Additionally, it is known that an increase in temperature, pressure, concentration of substrate and/or concentration of product as well as the amount and identity of catalyst present may affect the yield (conversion and selectivity) of hydrogenation reactions of glycolaldehyde. Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis, Shigeo Nishimura, ISBN: 978-0-471-39698-7, April 2001.
In summary, the efforts to provide an industrial scale process for hydrogenation of the fragmentation product composition of an industrial scale thermolytic fragmentation of sugars to produce ethylene glycol have shown to be challenging. Formaldehyde formed in the thermolytic fragmentation has shown to poison the hydrogenation catalyst, even at low concentrations. In addition, the reaction conditions have shown to unpredictably affect the selectivity, conversion rate and hydrogenation catalyst lifetime. Finally, formation of unwanted side products may complicate the subsequent purification of the hydrogenation product composition.
Consequently, there is still a need for improving the process of producing ethylene glycol from sugars via thermolytic fragmentation of sugars followed by catalytic hydrogenation of the resulting fragmentation product composition to avoid toxic compositions, obtain higher yields and higher selectivities and reduce the amount of undesirable side products at low costs to make it suitable for industrial scale production of ethylene glycol.